Conventionally, ethylene and propylene are produced via steam cracking of paraffinic feedstocks comprising ethane or ethane/propane mixtures, known as gas cracking, or propane, butane, naphtha, NGL (natural gas liquids), condensates, kero, gas oil and hydrowax, known as naphtha cracking. An alternative route to ethylene and propylene is an oxygenate-to-olefin (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can be converted into, for instance, methanol or dimethylether (DME), both of which are suitable feedstocks for an OTO process.
In an OTO process, an oxygenate such as methanol or dimethylether is provided to a reaction zone of a reactor comprising a suitable conversion catalyst and is converted to ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the oxygenate such as methanol is converted to higher hydrocarbons including C4+ olefins, paraffins and carbonaceous deposits on the catalyst. The catalyst is continuously regenerated to remove a portion of the carbonaceous deposits by methods known in the art, for example heating the catalyst with an oxygen-containing gas such as air or oxygen.
The effluent from the reactor, comprising the olefins, any unreacted oxygenates such as methanol and dimethylether and other reaction products such as water, once separated from the bulk of the catalyst, may then be treated to provide separate component streams. In order to increase the ethylene and propylene yield of the process, the C4+ olefins component stream may be recycled to the reaction zone or alternatively further cracked in a dedicated olefin cracking zone to produce further ethylene and propylene.
Following reaction, the reaction effluent stream must be cooled before being treated to provide separate component streams. Conventionally, the reaction effluent stream is cooled to around 140 to 350° C. using one or more heat exchangers, often one or more transfer line exchangers (TLEs), before being contacted with a cooled aqueous stream in a quench tower. A quench tower comprises at least one set of internals such as packing and/or trays. In usual operation, the gaseous stream to be quenched is fed into the quench tower below the internals and one or more cooled aqueous stream is fed into the quench tower above the internals. Thus, the gaseous stream travels upwards through the quench tower and is brought into contact with the one or more cooled aqueous stream travelling downwards through the tower (counter-currently to the gaseous stream). An aqueous stream containing condensed materials is removed at the bottom of the tower, cooled and recycled. The cooled gaseous stream is removed from the top of the quench tower.
Acidic by-products, such as formic acid and acetic acid, are formed in the reaction zone and are present in the reaction effluent stream. This formation of acidic by-products also continues as the reaction effluent stream is cooled to around 200° C. in one or more heat exchangers. As the gaseous reaction effluent stream is then further cooled, the components begin to condense to form liquids. The first drops of liquid formed are highly acidic (pH as low as 1 or 2), due to the presence of the acidic by-products. Such low pH material leads to corrosion of the process equipment. This corrosion is known as ‘dew point corrosion’ as it occurs at the dew point, which is the temperature at which a vapour in a volume of gas will condense into liquid, for a certain pressure.
Due to the high temperatures in the reaction zone and the acidity of the catalyst, a portion of the oxygenates such as methanol may unavoidably decompose thermally or catalytically into oxides of carbon, i.e. carbon monoxide and carbon dioxide in the gaseous form. Carbon dioxide generated during the OTO process is an acidic gas which is, thus, present in the effluent from the reactor. In order to prevent contamination of the olefinic product and problems associated with the formation of solid carbon dioxide during the separation of the olefinic product into olefinic component streams, which may be carried out at cryogenic temperatures, carbon dioxide should be removed from the reaction effluent and from the gaseous effluent from the cooling process before separation into olefinic component streams, for instance by treating with a caustic solution.
Carbonyl compounds, such as aldehydes and ketones, particularly formaldehyde and acetaldehyde, are commonly generated by the catalyst in side reactions and are also found in the effluent from the reactor. High CO concentrations and long residence times at temperatures in the range of from 100 to 350° C., particularly in the presence of metal surfaces, will further promote the formation and accumulation of aldehydes. Thus, in a conventional set-up, comprising one or more TLEs followed by a quench tower, a considerable amount of aldehyde production occurs in the pipes connecting the TLEs to the quench tower.
These carbonyl compounds may absorb and build up in the caustic solution used to remove carbon dioxide and other acid gases downstream of the quench section. The basic components of the caustic solution, such as hydroxide ions, can catalyse the aldol condensation and subsequent dehydration reactions of particularly acetaldehyde to form unsaturated aldehydes such as acrolein, especially at higher pH, such as a pH of greater than 9. Unsaturated aldehydes will polymerise when allowed to accumulate in the caustic solution and if the aldol condensation reaction is left unchecked, viscous oily polymers, polymer films and lumbs can be formed. These are known as ‘red oil’, are insoluble in the caustic solution and can deposit on equipment internals, causing severe fouling and blockages.
Catalyst fines are usually present in the reaction effluent stream, even after separation of the bulk of the catalyst. With continuous recycling of the aqueous streams (into which the fines will pass), any catalyst fines present in the reaction effluent stream build up on internals, causing blockages.
It would be desirable to avoid the problems of dew point corrosion, red oil make and catalyst fine blockages in a process for the preparation of olefinic products from oxygenates.